The invention relates to the post-extrusion processing of extruded profiles, such processing including blow molding the profile to a larger internal diameter while correspondingly imparting at least one bend into the expanded profile as well as optionally compression molding ends onto the profile, blow molding a second region of the profile into another shape, e.g., bellows or in-line check valve, and injection overmolding. In one embodiment, the process will include the bending of at least two angles into the profile, the angles being non-planar with respect to each other. The process involves heating at least a portion of the essentially linear extruded profile in a profile heating means to a first temperature at which the profile becomes formable or pliable or bendable, yet which still has at least some degree of structural integrity at this point which permits it to be physically manipulated without compressing the profile by the application of external pressure or by the weight of gravity itself. This preheating step is followed by additional heating to a second higher temperature at which the profile becomes melt processable and permitting radial expansion under pressure or melt fusion under pressure. In one specialized embodiment of this invention, the latent heat of extrusion retained by the extruded profile is sufficient to permit reformation of either segments of the tube, or the entire tube without the application of external heat, the reforming of at least portions of the tube being effected by the application of pressure, either internal to the profile or external to the profile.
There are various primary polymer processing technologies which are applicable in the manufacture of parts of various designs and shapes. Each technology as discussed below, has design constraints which limit its implementation in the fabrication of variously configured components.
Blow molding is a process typically used for the production of hollow thermoplastic components. The most widely known blow molded objects are bottles, jars, cans, and containers of all kinds for the food, beverage, cosmetic, medical, pharmaceutical and home products industries. Larger blown containers are often used for the packaging of chemicals, lubricants, and bulk materials. Among other-blow molded items are balls, bellows, and toys. For the automotive industry, fuel tanks, car bumpers, seat banks, center consoles, and armrest and headrest skins are blow molded.
The most prevalent blow molding grade plastic raw material is high-density polyethylene. Most of the milk jugs are made from this polymer. Other polyolefins, e.g., low density polyethylene, polypropylene, are also widely processed by blow molding. Depending on the application, styrenes, vinyls, polyesters, polyamides, polyurethanes, polycarbonates, and other thermoplastics are blow molded.
More than half of all blow molded parts are made by extrusion blow molding. The extrusion process is defined as making a product (extrudate) by forcing material through an orifice or die. The extrusion blow molding process consists of five steps: (1) extrusion of a plastic parison (hollow plastic tube); (2) closing of two mold halves on the parison, clamping the mold and cutting the parison; (3) blowing the parison against the cooled walls of the mold cavity, calibrating the opening, and holding it under air pressure during the cooling time; (4) opening the mold and removing the blown part; and (5) finishing the part by trimming off the flash.
A basic blow molding machine comprises an extruder, an extrusion head, a press section containing the mold, a calibration, a parison separation device, and an electrical control station. This fundamental unit is called a blow-and-drop machine. Plastic pellets are fed into the hopper mounted to the extruder. A motor-driven rotatable screw moves the material toward the blow molding or extrusion head and through the die.
Most extruders used in the blow molding are single-screw, either smooth-barrel or grooved barrel. Extruder output is determined by the geometry of the water-cooled feed zone and the feed capacity of the screw per revolution. With continued extrusion, a symmetrical or asymmetrical tube (parison) is formed by the die and pin in the extrusion head. An asymmetrical parison is developed by shaping or ovalizing the tooling in the head. Die and pin often move relative to each other during the extrusion process. This is caused by parison programming. Continuous extrusion is used with shuttle-type and wheel-type machines. Since the parison is extruded continuously, an open mold is positioned periodically around the parison. With the parison at its proper length, the mold is closed and clamped, and the parison is cut. Thereafter, the mold moves back under the calibration station, where the parison is blown into the shape dictated by the mold cavity. The neck of the bottle or container is calibrated simultaneously, in most cases by top blowing. Objects without openings are often needle-blown. Single-station shuttle machines have the mold positioned at the left or the right of the extrusion head while double-station machines have molds to the right and left of the head.
A blow molding machine's productivity is governed by its cycle time, 80% of which is cooling time. This long period of time is required for cooling the hot plastic material prior to demolding, to prevent post-warpage and dimensional distortion of the finished part. Modern molding machines are built-in stations for post-mold cooling. Articles are transferred out of the mold into post-cooling devices that basically consist of a cooling mold, i.e., mold without pinch-offs. Cooling is accomplished via liquid cooling in the shell of the mold, and CO2, refrigerated air, or nitrogen. cooling inside the container. Advantages are seen in shorter cycle times and control of the part distortion inherent in parts with asymmetric configuration and thick walls.
After leaving the cooling stations, containers are accepted by trim or punching devices for flash removal. Wall thickness control and minimal generation of flash avoid the problems associated with flash removal, e.g., the need to regrind and reprocess the flash and the possibility that its removal may expose seams, which in turn can contribute to container cracking and splitting on impact. Continuous blow molding machines can cost between $250,000 to $1.5 million and are typically run at parison extrusion rates of ten feet per minute using pressures of from between 80-125 psi. This process has significant inherent rate limitations.
The other type of blow molding is injection blow molding which is a two stage process for producing completely finished plastic containers. In the first stage, the plastic is injection molded into a preform cavity where the parison is formed. The neck finish of the container is molded, as well as the shape of the parison, as the plastic is injected around the core pin and into the preform. Temperature and conditioning of the parison takes place at this stage. The parison then is transferred via the core pin to the blow mold, and air is introduced through the core pin to blow the parison to the shape of the blow mold. The completed container is then transferred to the ejection station.
Injection blow molding offers a number of advantages: (1) it produces scrap-free, close tolerance, completely finished bottles that do not require any secondary operations; (2) it offers positive weight control in the finished container; (3) neck shapes and finishes, internally and externally can be molded with accuracy; (4) repeatable weight and bottle dimensions are possible with the process; (5) improved clarity and strength due to the effect of some amount of biaxial orientation; (6) bottles are controlled and oriented at the ejection station; and (7) there is a minimum of operator supervision required. There are however, limitations to this process, relating primarily to the sizes and shapes of bottles that can be produced profitably on existing injection blow molding machines.
Compression molding has been used for such thermosetting compounds are urea, phenolic, epoxy, melamines and rubber. The most apparent advantage of compression molding of thermosets is the simple system involved. The material is placed in a heated cavity and is pressurized for the required cure time. Tooling costs are inexpensive because of the simplicity. Material is not wasted because of the absence of sprues and runners. Consistency of the part size is good and the absence of gate and flow marks reduce finishing costs. Depending on the part and material, positive, semi-positive and closed molds are used.
The compression molding press two platens that close together, applying heat and pressure to mold material into the wanted shape. Most compression presses are hydraulically operated. Heating of the molds can be done for shallow parts by using cartridge or strip heaters in the platen. Deeper parts need electrical cartridge-type heaters in the platen or require steam or hot-oil systems.
Plastics extrusion processing is defined as converting plastic powder or granules into a continuous uniform melt and forcing this melt through a die which yields a desired shape. This melted material must then be cooled back to its solid state as it is held in the desired shape, so an end product can be realized.
Single screw extruders are the most common in use today. Extruders diameters range from ½″ to 12″ in a barrel inner diameter. The hopper of an extruder accepts granules or powder which pass through a vertical opening in the feed section where they are introduced to a rotating screw with spiral flights. The material is conveyed along the screw and heated inside the barrel, with the goal being to reach the die system in a totally melt phase at an acceptable and homogeneous temperature, and being pumped at a consistent output rate.
The barrel is heated and cooled by heater/coolerjackets surrounding its outer wall to aid in the melting of the material on the screw. Heater/coolers are electrically heated through heating elements cast into aluminum, with either cooling tubes also cast into the aluminum or deep fins cast on the outer surfaces of the heaters/coolers to allow air cooling of the barrel via blowers. Temperature of the various barrel zones are set according to the material, screw design, and processing goals. These barrel zone temperature settings vary widely, depending on the material used or the product being made while the control of the temperature at the deep barrel thermocouple position for a given situation is typically maintained within a close tolerance range to minimize variations of material exiting the die system. The screw is the heart of the extrusion process and designs for which have varied with time as understanding of the melting process of the plastic material moving along the screw has increased. Since some materials tend to trap air as they start to melt, or contain moisture or volatiles, that will create porosity in the final product, a vent is typically positioned at a point in the barrel to remove the porosity by allowing the escape of gases.
The melt must be shaped and cooled by product sizing and cooling equipment to its solid phase while forming a product that falls within given size tolerances. The dies to create the end products from a melt are varied depending on the shapes involved. Pipe and tubing are cooled through simple, open water troughs, or pulled through vacuum sizing tanks, where the melt is held in a sizing sleeve of a short time in a water filled vacuum chamber. Custom profiles come in various shapes and are commonly made of materials that have high melt viscosity, so they are easy to hold shape while they cool. These products can be cooled by forced air, water troughs, or water spray methods. The methods of getting the many shapes include various sizing fixtures to hold the extrudate as it is pulled through the system and cooled. The material can also be coextruded, i.e., made with more than one material. Coextrusion typically requires a dual-extrusion head and multiple extruders using a specialized die system to bring these layers together with a common sizing and shaping system. Extruders can cost from $20,000 to $300,000 depending on size and options and the process is generally not rate limited as is the case with blow molding. Rates of 100 feet per minute are routinely achieved.
Injection molding of thermoplastics is a process by which plastic is melted and injected into a mold cavity void, defined in this instance as the void volume between the mold core body and the mold cavity. Once the melted plastic is in the mold, it cools to a shape that reflects the form of the cavity. The resulting part is a finished part needing no other work before assembly into or use as a finished part. The injection molding machine has two basic components: an injection unit to melt and transfer the plastic into the mold; and a clamp to hold the mold shut against injection pressures and for parts removal. The injection unit melts the plastic before it is injected into the mold, then injects the melt with controlled pressure and rate into the mold. After the injection cycle, the clamp gently opens the mold halves.
Important factors in the processing of plastic include temperature, consistency, color dispersion and density of the melt. Conductive heat supplied by barrel temperature and mechanical heat generated by screw rotation both contribute to the processing of good quality melt. Often, most of the energy available for melting the plastic is supplied by screw rotation. Mixing happens between screw flights and the screw rotates, smearing the melted surface from the plastic pellet. This mixing/shearing action is repeated as the material moves along the screw until the plastic is completely melted.
If the polymer is a thermoset, injection molding uses a screw or a plunger to feed the polymer through a heated barrel to decrease its viscosity, followed by injection into a heated mold. Once the material fills the mold, it is held under pressure while chemical crosslinking occurs to make the polymer hard. The cured part is then ejected from the mold while at the elevated temperature and cannot be reformed or remelted.
When thermoplastics are heated in an injection press, they soften and as pressure is applied, flow from the nozzle of the press into an injection mold. The mold has cavities that, when filled with the thermoplastic or thermoformable material, define the molded part. The material enters these cavities through passages cut into the mold, called runners. The mold also has passages in it to circulate a coolant, usually water, through strategic areas to chill the hot plastic. As it cools, the thermoplastic material hardens. When cooled enough, the mold opens and the part is removed.
This means that during an overmolding process, the polymeric material used must be sufficiently formable, by melting, such that it may be forced to flow into and around the other preformed element(s)(e.g., a core insert, often polymeric in nature and/or polymer tube(s)). During this thermomelting process, heat and pressure are often applied.
To date, there has been no technology described which combines the features of extrusion, blow molding, compression molding and injection molding. The Prior Art has typically taught the need to pick and choose between various technologies. The invention described and discussed herein teaches a method of manufacturing a blow molded part using a predefined length of extruded profile as the raw material. The value of this approach is that the inherent rate limitations of blow molding are overcome as well as the size and shape restrictions on the part that can be manufactured. One of the unique aspects of the technology is the ability to take a part that is too long or too complex to blow mold and to add this to an extruded profile in a specific area. This approach is not limited to extruded profiles which are tubular in nature, but rather works with any hollow portion of the extruded profile. By using non-uniform heating, it is possible to expand or contract the profile in a given area, thereby creating additional novel features in the extruded profile, not possible using either extrusion or blow molding technologies in isolation. Additionally, by varying the extrusion rate of the profile, thicker and thinner profile regions can be created, these regions being targeted for subsequent post-processing.